Some geothermal sources produce two-phase fluids with a pressure around 800 psia comprising saturated steam, hot brine, and toxic non-condensable gases such as hydrogen sulfide. In order to produce electricity using the hot fluids as an energy source, an arrangement such as that shown and described in the above-identified '156 application can be utilized. In such an arrangement, the geothermal fluid emerging from the ground is separated into two streams: a gaseous stream containing steam and the non-condensable gases; and a liquid stream containing hot, corrosive brine. For reasons of increased reliability and ease in fabrication of components and construction of power plant installations at remote locations near the geothermal source, the power plant is constructed as-a plurality of identical modules each of which includes a steam turbine and an organic vapor turbine directly coupled to a single electric generator.
The gaseous stream at the output of a separator located at the production well is applied, in parallel, to the steam turbines of each module where expansion takes place producing work that is converted by the generator into electricity, with low pressure steam that exits the turbine after expansion takes place being applied to a steam condenser in the module. The steam condenser, which contains liquid organic fluid, such as pentane, functions also as a vaporizer in that the latent heat of condensation of the steam released as the steam condenses, serves to vaporize the organic fluid. Vaporized organic fluid is applied to the organic vapor turbine wherein expansion takes place producing work that is converted by the generator into electricity, the low pressure organic vapor exiting the turbine after expansion takes place being applied to a vapor condenser in the module. Usually, the vapor condenser is air-cooled; and the resultant liquid organic fluid produced passes through a preheater before being returned to the steam condenser. The preheater is also supplied with steam condensate produced by the steam condenser, and the cooled steam condensate is then returned to an injection well.
Because of the large amount of noncondensable gases present in the gaseous stream, the steam condenser is tapped for these gases which are returned with the cooled steam condensate to the injection well. Finally, because the liquid stream produced by the separator is so corrosive, the brine that constitutes the liquid stream is conveyed directly to the injection well without being involved in a heat exchange process.
While this approach works reasonably well in safely containing the non-condensables, and in avoiding the corrosive effects of the brine, the failure to utilize the heat contained in the brine wastes a sizable percentage of the heat contained in the geothermal fluid extracted from the ground. In addition, there are other problems, mainly concerning the large amounts of hydrogen sulfide that are always present in the geothermal fluid. First of all, because separation of the hydrogen sulfide occurs in the steam condenser, wherein the pressure is about one atmosphere, compression of the hydrogen sulfide is required not only to inject the gas into the steam condensate and to force the mixture into the ground, but also to ensure the solubility of the gas in the liquid being injected.
The second problem arising from the presence of hydrogen sulfide in the steam applied to the turbine is the necessity for protecting the turbine blades from corrosion by using a special coating, for example. In addition, both the turbine seals and the compressor seals are not leak proof with the result that some hydrogen sulfide leaks through the seals, a situation that is dangerous to those in the vicinity of the power plant, and detrimental to the environment. Furthermore, the amount of hydrogen sulfide in the geothermal fluid is likely to vary with time, and this places additional mechanical strain on the hydrogen sulfide compressor as the amount of work varies thereby increasing maintenance problems.
In addition to all of these problems, is a problem of safely controlling the two-phase flow in the well in the face of required changes in flow due to power load changes and periodic shut downs for maintenance of the turbines and generator. Rapid changes in the mass flow of a two-phase flow system are difficult to control and involve the risk of blowing out the control valve due to mechanical and thermal shocks introduced into the piping by rapid flow changes.
It is therefore and object of the present invention to provide a new and improved method of and apparatus for producing work from a two-phase, high pressure geothermal fluid which ameliorates the problems described above.